The dust produced as a result of electric furnace steel making processes has been classified by the U.S. Environmental Protection Agency as "Hazardous Waste". Accordingly, this material must be manifested for transport and disposed only in a limited number of licensed hazardous waste disposal sites. This is costly, currently in excess of $100/ton. As the number of available sites diminishes, clearly the disposal costs will inexorably rise.
Unfortunately, proposed alternatives are generally impractical. The world alloy steel business is based upon electric arc furnace melting, with the inevitable production of fume. To add insult to injury, the ingredient value of the fume is usually so low as to preclude economic recovery of materials entrained in the fume.
Typical dust arising from the production of alloy steel has the following representative composition (in weight percent):
Fe--25.0-35.0 PA1 Ni--0.5-1.5 PA1 Cr--0.5-2.0 PA1 Mn--2.0-4.0 PA1 Zn--4.0-10.0 PA1 Pb--1.0-5.0 PA1 Cd--0.1-1.0 PA1 Al.sub.2 O.sub.3 --0.5-2.5 PA1 CaO--3.0-12.0 PA1 SiO.sub.2 --02 3.0-15.0 PA1 MgO--1.0-5.0 PA1 Chromium--5 mg/l PA1 Lead--5 mg/l PA1 Cadmium--1 mg/l PA1 Arsenic--5 mg/l
Dust from the increasingly popular mini-mills is similar but usually contains even lower levels of nickel and chromium and higher zinc--up to 30% zinc.
The hazardous nature of electric furnace oxide fume is caused by the generation of soluble oxides of Pb, Cd, As and Cr.sup.6+ formed at high temperatures above the steel bath and in the off-gas system of the electric arc furnace. After the dust is disposed in landfills, the soluble oxides find their way into the water table causing pollution.
The Environmental Protection Agency has specified a toxicity test using an accelerated leach with diluted acetic acid. Toxicity is then determined as milligrams of listed metals per liter of leachate. Current listed metals and their test limits are given below:
The potentially valuable elements in the dust such as nickel, chromium, zinc and iron are present at levels lower than those of the unimportant materials and cannot currently be recovered economically.
In particular there are a number of techniques that may be used to recover selected components. For example, the SKF Plasmadust.TM. process is designed to recover zinc. However, it does hot appear to be economically viable since high zinc dust generators are widely separated and the current processing and shipping costs of the recovered zinc vis-a-vis the current cost of zinc is not attractive. Another recovery alternative is taught by U.S. Pat. 3,836,353. Iron and oxide impurities are recovered by the volatilization of zinc, lead and sulfur impurities and the subsequent formation of iron oxide containing pellets in a rotary hearth furnace. Iron oxide prices are similarly depressed.
From the foregoing, it appears that a low cost process to render these listed wastes harmless and simply return them safely to the earth from whence they came and without the concern for element recovery is desirable.